change equal values in a column to nan

Question

FC93 2017-2-6

0
链接

此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/323660-change-equal-values-in-a-column-to-nan

评论： John Chilleri 2017-2-8

I have matrix with several columns and rows. Most of the columns are like (1.02 1.05 1.03 1.04 1.04 1.04 1.04 .. 1.04). No I would like to search in my matrix in each column the point from where the following numbers in the column are equal change the second until the last value to nan.

As an example I would like to change (1.02 1.05 1.03 1.04 1.04 1.04 1.04 .. 1.04) to (1.02 1.05 1.03 1.04 nan nan nan ... nan).

But there are some columns that don't have a point from where the column has the same value until the end.

Thank you for your help.

0 个评论
显示 -2更早的评论隐藏 -2更早的评论

请先登录，再进行评论。

请先登录，再回答此问题。

Answer 1

John Chilleri 2017-2-6

1
链接

此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/323660-change-equal-values-in-a-column-to-nan#answer_253586

编辑：John Chilleri 2017-2-8

在 MATLAB Online 中打开

Hello,

Try this:

% Given matrix A
for i = 1:size(A,2) % go through each column
   for j = 2:size(A,1) % through elements in column
      if (sum(A(j-1,i) == A(j:end,i)) == size(A,1)-j+1)
         A(j:end,i) = NaN;
         break;
      end
   end
end

If you have any questions please ask! This will only replace the values if they're forever equivalent (if it repeats 2,2,2,3, it wont replace the 2,2,2).

Hope this helps!

3 个评论
显示 1更早的评论隐藏 1更早的评论

Jan 2017-2-8

在 MATLAB Online 中打开

The comparison

if (sum(A(j-1,i) == A(j:end,i)) == length(A(j:end,i)))

is more expensive than required. The length of A(j:end,i) is size(A,1)-j+1. The explicit creation of A(j:end,i) only to determine its length, consumes time.

A simplification:

   for j = size(A,1)-1:1 % through elements in column
      if A(j,i) == A(j+1,i)
         A(j+1,i) = NaN;
      else
         break;
      end
   end